add an example on how to use lenses

[legrostdg]

I can't get how to use the lenses generated by purescript-bridge...

Let's suppose I have the following haskell types defined:

data Book = Book { author :: Author, pages :: Int }
data Author = Author { name :: String }

given a purescript Book:

let a = { name: "James" }
let b = { author: a, pages: 100}

How can I access to the number of pages, and the author name?
I would expect something like this:

let pages = b ^. _pages
let name = b ^. _author ^. _name

but it does not seem to work...

[eskimor]

lenses are only generated if you prefix your fields with _, just like in haskell. So your record should be:

_author :: Author

and the lens access:

let pages = b^.pages

[legrostdg]

So, I have to use lenses with haskell if I want to use them with purescript?

[eskimor]

Actually no, but yes the intention is to have a more seamless translation. If you had lenses in Haskell you have them in PureScript too.

No because you don't have to have lenses in haskell, all you need is to prefix your field names with an underscore.

Alternatively if you really want lenses and prefixes in PS but not in Haskell you can add the prefixes by providing your own custom bridge.

[safareli]

what do you mean by "providing your own custom bridge"?
like if you have data Foo = Foo {fooName:: String} in haskell, what can be done so in generated code you have data Foo = Foo {name:: String}

[eskimor]

Hehe, sorry that suggestion did not receive too much thought ;-) You are right, you can't change record field names by means of a custom bridge, as bridges only translate types. If you want something like this, you would need to modify the SumTypes accordingly before passing them to writePSTypes. Check out RecordEntry in DataConstructor in SumType.

Simply modify the _recLabels accordingly. One thing though, if you do translations like this, the generic encodings will be incompatible by default, so you will have to take care of that too.

I hope that helps!

[safareli]

Thanks!

[peterbecich]

Here is an example using view on generated Purescript types:

purescript-bridge/example/src/Main.purs

Lines 48 to 51 in b025797

	<> "\t Foo list length: "
	<> (show (length $ view fooList foo :: Int))
	<> "\t Foo map size: "
	<> (show (Object.size $ view fooMap foo :: Int))